A modular weapon system will almost always include a rail system. Integrated rail systems enable fine-tuning of a firearm with modifications and variants including different types of optics, scopes, sights, lights, secondary weapons, grips, etc. A modern rail attachment system is typically incorporated into the frame of the weapon and provides flexibility for placement of the different components on the weapon. NATO STANAG 4694 and MIL-STD-1913 (i.e., Picatinny) document rail system standards that are used industry-wide. The Weaver rail is also a common interface system.
Common modular components used in configurable weapons include a brace and a gunstock. A brace is typically recognized as a device mounted to a pistol or short barrel handgun that helps improve aiming and support when shooting the firearm. A brace will typically fit over a recoil buffer used to reduce or cushion the impact to a shooter of recoiling parts of the gun. For example, a brace in U.S. Pat. No. 8,869,444 is described as engaging a shooter's forearm to secure a rearward end of the handgun frame. Optionally, a pistol brace can be designed to rest against a user's cheek, upper arm, or shoulder to provide stability.
A gunstock, stock, buttstock, or butt is attached to a firing mechanism of a long gun such as a rifle and is held against the user's shoulder when shooting the gun. The stock allows the shooter to firmly support the weapon and to easily aim the weapon with stability, and also transmits recoil into the shooter's body. A stock can include a front portion or fore-end attached to the receiver that relays recoil from the barrel. A butt portion of the stock typically rests against the shooter's shoulder to provide stability. Optionally, a shooter may rest their cheek against the stock to improve aiming and stability.
Stocks can be one-piece or two-piece designs. A one-piece stock is a single unit from butt to fore-end, such as that commonly found on a conventional bolt-action rifle. Two-piece stocks use separate pieces as the butt and fore-end, such as that commonly found on break-action shotguns, and lever-action rifles and shotguns. A stock can include a grip held by the shooter during firing and a comb that supports the shooter's cheek, which provides alignment when aiming and stability when firing. Buttstocks can be made to be rigidly fixed or foldable and can include adjustment features used to customize the geometry to fit an individual shooter's comfort or mission situation.
FIG. 1 shows a modularized weapon of the related art. As shown in FIG. 1, the weapon includes no brace or buttstock. Instead, the weapon of FIG. 1 includes an end cap 1 at the aft or proximal end of the weapon 2. As shown, the end cap 1 seals the end of the weapon 2, retains the rear portion of the recoil buffer mechanism, and includes a swivel adapter 3 that provides a sling attachment location. In this design, the weapon would need to be broken down (disassembled), and the end cap 1 removed, before replacing the end cap 1 with a brace or one-piece buttstock. This operation takes time and would be difficult to perform in the field. Additionally, as there is no standard interface at the rear of such weapons, each brace or buttstock must be specifically designed to mate with each corresponding modular weapon platform.